A grey water interface valve (GWIV) system may be installed below a sink in a lavatory or galley of a passenger transport vehicle, such as an aircraft. The GWIV system may be provided in connection with a reservoir that serves as a holding tank for grey water or other liquids leaving the sink. The main components of a GWIV system include a wash basin/sink, a reservoir, a control unit, and a valve. Grey water (which can include spent water from hand-washing, detergents, unconsumed liquid beverages, or any other combination of liquids or semi-solid material) may leave the wash basin drain and flow into the reservoir. The reservoir may be in fluid communication with a vacuum line of the vehicle (or other location for the intended re-use of the grey water, if applicable). When the reservoir is full, most (but not all) of the water should be evacuated out of the reservoir. A small amount of water is generally allowed to remain in the reservoir in order to maintain a noise buffer between the vacuum line and the wash basin.
The control unit of this system may have one or more sensors that detect when the reservoir is full or should otherwise be emptied. However, improvements to sensors for detecting liquid movement through the GWIV system are desired.